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sfxge: add function to query link control privilege

Description

sfxge: add function to query link control privilege

Make link control privilege visible to OS driver to guard updates to
flow control and PHY advertised capabilities.

Submitted by: Richard Houldsworth <rhouldsworth at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D4330